A basic structure of a magnetic tunnel junction (MTJ) element serving as a magnetoresistive element is a stacked structure including a storage layer, the magnetization direction of which is changeable, a reference layer, the magnetization direction of which is fixed, and an insulating layer provided between the storage layer and the reference layer. It is known that an MTJ element shows a tunneling magnetoresistive (TMR) effect. Thus, an MTJ element is used as a storage element of a memory cell in a magnetic random access memory (MRAM).
An MRAM stores information (“1”, “0”) in accordance with a change in relative angle between magnetizations of magnetic layers included in an MTJ element, and is nonvolatile. Since the magnetization switching speed of an MRAM is a few nanoseconds, it is possible to write and read data at a high speed with an MRAM. Accordingly, MRAMs are expected to be next-generation hig h-speed nonvolatile memories. Furthermore, by using a method called “spin transfer torque switching” for controlling magnetization by means of a spin polarized current, a current density can be increased by decreasing the cell size of an MRAM. Accordingly, it is possible to easily switch the magnetization in a storage layer, resulting in that it is possible to obtain an MRAM with a high density and a low power consumption.
In improving the density of a nonvolatile memory, it is essential to improve the degree of integration of a magnetoresistive element. However, as the size of a magnetoresistive element decreases, the thermal stability of a ferromagnetic material for forming the magnetoresistive element deteriorates. Therefore, a problem arises in that the magnetic anisotropy energy and the thermal stability of the ferromagnetic material should be improved.
In order to solve this problem, recently, it is attempted that an MRAM using a perpendicular magnetization MTJ element, in which the direction of the magnetization of a ferromagnetic material is perpendicular to the film plane, be formed. In a perpendicular magnetization MTJ element, generally a ferromagnetic material with a high magnetic crystalline anisotropy is used.
Generally, a critical current for magnetization reversal by the spin transfer torque switching method is dependent on the saturation magnetization and the Gilbert damping constant of a storage layer. Accordingly, in order to switch the magnetization of a storage layer by means of low-current spin transfer torque switching, it is necessary to reduce the saturation magnetization and the Gilbert damping constant of the storage layer. Furthermore, the influence of strayed magnetic field from a reference layer becomes more remarkable as the element is miniaturized. Accordingly, in order to improve the degree of integration of a magnetoresistive element, it is also necessary to reduce the saturation magnetization of the reference layer thereof with a high magnetic anisotropy energy of the reference layer being maintained.